Additional drink, soft and clear, having no head (13)
I believe the answer is:
supplementary
'additional' is the definition.
(I've seen this before)
'drink soft and clear having no head' is the wordplay.
I cannot quite understand how this works, but
'drink' could be 'sup' (to sup is to drink) and 'sup' is present in the answer.
'soft' could be 'p' (musical abbreviation for 'piano') and 'p' is present in the answer.
'and' could be 'n' (common abbreviation for 'and') and 'n' is located in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for supplementary that I've seen before include "Additional" , "Ancillary" , "Added to complete or make up a deficiency" , "Extra" .)